The structural elements form a regular line screen. Each structural element usually has a center line with respect to which the width of the particular structural element is modulated to render a desired tonal value of the halftone image. The wider a section of the structural element is, the darker the represented tonal value appears.
In EP 0 710 574 A2 this type of halftone image representation is used as an authenticity feature for the purpose of copy protection of papers of value by having the spacing of the structural elements, that is, the spacing between their center lines, not be constant throughout the image but vary in accordance with a modulation function. This makes the spacing correspond at least in partial areas of the halftone image to the scanning grid of a copier or scanner used for duplicating the paper of value, causing a striking moiré pattern to arise in the corresponding partial area of the produced copy.